The present invention relates to solid electrolytic capacitors and methods for manufacturing the same.
In recent years, there has been a considerable increase in frequencies and electrical currents in integrated circuits of electronic appliances using solid electrolytic capacitors. There is thus a need for solid electrolytic capacitors with low equivalent series resistance (abbreviated as xe2x80x9cESRxe2x80x9d below), large capacitance, and small loss.
The following explains an example of a conventional method for manufacturing a solid electrolytic capacitor""s internal electrodes (that is, a capacitor element). First, a valve metal (for example tantalum) serving as an anode conductor is anodized in an electrolytic solution such as phosphoric acid, and an oxide film layer (dielectric layer) is formed on its surface. Then, a solid electrolyte is formed on the surface of this oxide film layer. A known solid electrolyte is, for example, manganese dioxide, which can be formed by immersing the anode conductor in a manganese nitrate solution, retrieving it, and calcining it. Finally, a cathode conductor is formed on the solid electrolyte. For the cathode conductor, it is possible to use a laminate of a carbon layer and an outer silver/conductive resin layer. In order to connect the capacitor element electrically to the outside, an anode lead terminal is connected to the anode conductor, and a cathode lead terminal is connected to the cathode conductor.
The resistances of all the respective parts affect the ESR, but what needs to be considered most for the resistance and opens room for improvement is the solid electrolyte. In order to reduce the resistance of the solid electrolyte, it has been proposed to use a conductive polymer material with a conductivity that is higher than that of manganese dioxide (whose conductivity is about 0.1 S/cm), and this also has been put to practice. For example, it is possible to realize a conductivity of about 100 S/cm using polypyrrole. As monomers other than pyrrole for forming the conductive polymer material, anilines, thiophenes and 3,4-ethylenedioxythiophene are known for example. The methods for forming the conductive polymer layer can be divided broadly into chemical oxidative polymerization and electrolytic oxidative polymerization.
The ESR also is affected by the contact resistance between the layers. JP 2000-232036A by the applicant of this application discloses the mixing of conductive polymer particles into the conductive polymer layer, and lowering the contact resistance between the conductive polymer layer and the cathode conductor with the irregularities formed by these particles. In the method described in this publication, the conductive polymer layer is formed by chemical oxidative polymerization using a polymerization solution in which conductive polymer particles are dispersed.
In order to increase the capacitance of the capacitor, it also has been proposed to form a conductive polymer layer in form of particles. JP H8-45790A discloses that a polypyrrole made of particles with a particle diameter of not more than 0.2 xcexcm is formed by chemical oxidative polymerization using a polymerization solution in which the mol ratio at which the monomers are mixed with the oxidizer is at least 1. If the particle diameter of the conductive polymer layer is kept small, then peeling of that layer can be suppressed, and the latent capacitance of the dielectric layer can be utilized more easily.
It is known that conductive polymer layers formed by electrolytic oxidative polymerization have higher conductivity and better film properties than conductive polymer layers formed by chemical oxidative polymerization. However, when electrolytic oxidative polymerization is carried out repeatedly with a single electrolytic liquid, then the conductivity of the conductive polymer film changes gradually. JP H-11-121279A discloses an electrolytic oxidative polymerization that is carried out while keeping the pH of the polymerization solution within a predetermined range, in order to suppress this change.
JP 2000-297142A discloses that the pH of a polymerization solution used for electrolytic oxidative polymerization is set to 5 or less. Here, it is attempted to increase the rate of the polymerization reaction by reducing the pH.
Thus, a large number of solid electrolytic capacitors having a conductive polymer layer as the solid electrolyte have been studied in depth. However, a solid electrolytic capacitor that has both low ESR and high capacitance, and moreover exhibits low loss, has not been achieved satisfactorily so far.
In order to address these problems, according to the present invention, a method for manufacturing a solid electrolytic capacitor including an anode conductor that is made of a valve metal, a dielectric layer that is formed on a surface of the anode conductor, and a solid electrolyte that is formed on a surface of the dielectric layer and includes a conductive polymer layer, includes a step of forming, in a first solution, a first conductive polymer film serving as a portion of the conductive polymer layer; and a step of forming, in a second solution whose pH is lower than the pH of the first solution, a second conductive polymer film serving as another portion of the conductive polymer layer; wherein the first conductive polymer film and the second conductive polymer film are both formed by electrolytic oxidative polymerization. It is also possible to form the first and the second conductive polymer film by chemical oxidative polymerization instead of electrolytic oxidative polymerization.
In accordance with the present invention, a solid electrolytic capacitor includes an anode conductor that is made of a valve metal; a dielectric layer that is formed on a surface of the anode conductor; and a solid electrolyte that is formed on a surface of the dielectric layer and includes a conductive polymer layer. The anode conductor includes a plurality of pores. The conductive polymer layer comprises a first conductive polymer film made of a plurality of particles, and a second conductive polymer film having an average particle diameter that is larger than the average particle diameter of the plurality of particles. The second conductive polymer film is formed such that it covers the plurality of pores. The first conductive polymer film is formed such that at least a portion thereof is disposed inside the plurality of pores, or the first conductive polymer film is arranged as an outermost film of the conductive polymer layer.